Mobile data traffic is projected to grow at a phenomenal rate in the years to come. To cope with such growth, cellular network operators and equipment vendors are exploring various technologies to significantly improve network capacity. Utilization of more radio spectrum, heterogeneous network (HetNet) deployment, cell site densification, and coordinated multiple-point (CoMP) transmission and reception are among the ones that are currently being explored.
Regarding HetNet, there is currently an ongoing 3GPP study item for investigating potential opportunities to further enhance the performance of HetNet deployment in a Universal Mobile Telecommunications System (UMTS) network. In a HetNet, in addition to the placements of macro base stations, several micro/pico/femto/relay base stations and remote radio units (RRUs) are deployed within the macro cell coverage area.
The power transmitted by these micro/pico/femto/relay base stations and RRUs is relatively small compared to that of the macro base stations. These low power nodes (LPNs) are typically deployed to eliminate coverage holes in the homogeneous network (using macro base stations only). The LPNs can improve capacity in hot-spots. Due to their low transmit power and small physical size, the LPNs can offer flexible site acquisitions. In this document, the term low power node will be used generally to refer to a micro/pico/femto/relay base station (BS), or a RRU.
HetNets can be divided into two deployment categories—co-channel deployment and combined cell (or soft cell or shared cell) deployment. In the co-channel deployment, LPNs are deployed within a macro cell coverage area, where the transmission/reception points created by the LPNs have different cell IDs from that of the macro cell. This will also be referred to as separate-cell scenario. In the combined cell deployment, the transmission/reception points created by the LPNs share the same cell ID as that of the macro cell.
Challenges associated with the co-channel deployment motivate movement to the combined-cell deployment, in which a LPN functions as a cell portion rather than an individual cell.
The baseline implementation of combined-cell that works with any legacy User Equipment (UE) is single frequency network (SFN) operation. With SFN, all nodes within a combined cell transmit the same waveform. This means that an orthogonal variable spreading factor (OVSF) code can only be used to serve one UE in the same transmission time interval (TTI).
At a high network load, the downlink performance of a combined cell deployment based on SFN operation is inferior to that achieved by an equivalent separate-cell deployment with the same LPN locations and power levels. This is due to lack of spatial reuse within the combined cell.
In High Speed Packet Access (HSPA) operation, High Speed Shared Control Channel (HS-SCCH), which is a downlink control channel, is used to signal which UE is scheduled and also what transport format (precoder, code allocation, modulation, and coding rate) will be used on the associated data channel; High Speed Physical Downlink Shared Channel (HS-PDSCH). HS-PDSCH is the physical layer channel of High Speed Downlink Shared Channel (HS-DSCH). In this description, HS-PDSCH and HS-DSCH will be used interchangeably.
According to the 3GPP specifications, a reference channel (for the purpose of channel estimation) used for receiving the HS-SCCH is the Primary Common Pilot Channel (P-CPICH). Since the P-CPICH is transmitted using SFN in a combined cell, HS-SCCH needs to be transmitted using the SFN, to be consistent with its channel reference. The same HS-SCCH signal is transmitted in all cell portions.
To enable spatial reuse, multiple HS-SCCHs need to be transmitted in each cell portion. Thus, when there are many cell portions in a combined cell and spatial reuse is applied to many of these cell portions, HS-SCCH may consume a significant portion of the OVSF code tree.